Various types of combustion may be used in an internal combustion engine. For example, spark ignition (SI) of a homogenous mixture during the expansion stroke is one example method. This method relies on a timed spark from a sparking plug in order to achieve ignition within the combustion chamber of an air and fuel mixture. Another type of combustion may be referred to as homogeneous charge compression ignition (HCCI), which occurs when the temperature of the combustion chamber exceeds the required conditions for the specific fuel resulting in autoignition. HCCI can be used to provide greater fuel efficiency and reduced NOx production under some conditions. One example of such operation is described in U.S. Pat. No. 6,390,054.
The inventors herein have recognized that one issue with such autoignition-type operation is that the timing of combustion can be affected by many parameters. For example, the timing of combustion may be affected by charge temperature, air-fuel ratio, engine speed, and/or engine load. Specifically, small variations in such parameters can result in auto-ignition occurring too early, or too late. Such variation can increase emissions and reduce fuel savings, thereby degrading performance.
As such, the inventors herein have also recognized that, when trying to purge fuel vapors into the engine, one particular way in which combustion timing may be inadvertently affected is due to uncertainty in the concentration and/or amount of fuel vapors being purged into the combustion chamber. In other words, during fuel vapor purging operations, recycled fuel vapors may substantially increase the variability of fuel temperature, atomization and/or air-fuel ratio, thus exacerbating degradation of ignition timing control. One approach is to avoid fuel vapor purging during auto-ignition operation, and discontinue such operation when it is necessary to purge fuel vapors; however, this can limit the available time to operating with auto-ignition.
In another approach, the above issues may be addressed by a method of operating an internal combustion engine having a combustion chamber with a piston, comprising: inducting at least air and fuel vapors from a fuel vapor system; and adjusting an operating condition of the engine so that a mixture of air and fuel in the combustion chamber, including said fuel vapor, approaches, but does not achieve, said autoignition temperature; and performing a spark from the spark plug so that said mixture substantially auto-ignites.
In this way, it is possible, during at least some periods of fuel vapor purging, to utilize a spark-assisted autoignition, and thereby still maintain accurate control of combustion timing while also purging fuel vapors. Thus, additional auto-ignition operation can be realized, while still providing sufficient opportunity to purge fuel vapors. Note that spark-assisted auto-ignition may be used without fuel vapor purging as well, if desired.